The present invention relates to a polyester resin which exhibits improved processability during manufacture into biaxially-oriented polyester film. More particularly, the present invention relates to a polyester resin which contains a copolyesterether which is based upon 1,4-cyclohexanedimethanol.
Polyester resin is typically manufactured into film by extruding melted polyester resin through a slot die onto a casting drum, thereby rapidly cooling the polyester to form an amorphous (non-crystalline) sheet. The polyester sheet is then biaxially oriented at elevated temperature This is typically accomplished by first stretching the polyester sheet in the longitudinal direction to form monoaxially-oriented polyester film, and then stretching the polyester film in the transverse direction The biaxially-oriented film is then thermally treated under dimensional restraint ("heat-set") to crystallize the film. The process summarized above is described in U.S. Pat. No. 2,823,421.
One problem which can seriously affect the profitable production of polyester film is the frequent and random occurrence of film splits during transverse direction stretching of the film. The frequency of such splitting can depend on a variety of factors including (1) the chemical composition of the film, (2) low melt viscosity (i.e. low average molecular weight), (3) the thickness of the film and (4) the processing conditions, such as temperature, stretching ratios, and stretching rate.
One solution to the problem of excessive film splitting is to increase the average molecular weight of the polyester resin, which is typically reported as its intrinsic viscosity. For example, increasing the intrinsic viscosity of the polyester resin from 0.65 to 0.85 deciliters/gram will substantially reduce film splitting. However, increasing the average molecular weight of the resin is expensive and can result in other processing difficulties. For example, higher molecular weight resins typically require undesirable higher extruder pressures.
The copolyesterethers which form part of the present invention are known per se. U.S. patent No. 4,349,469 discloses elastomeric copolyesterethers based upon poly(1,4-cyclohexanedimethylene-1,4-cyclohexane dicarboxylate) which has been modified with polytetramethyleneether glycol. The copolyesterethers so produced exhibit good melt strength and may be extrusion blown to make clear plastic bags or film. More recently, U.S. Pat. No. 4,636,442 disclosed the coextrusion of the same copolyesterether with a layer of polyethylene terephthalate to produce biaxially-oriented film having superior resistance to flex cracking. U.S. Pat. No. 4,604,446 discloses a sulfonated copolyesterether which is suitable for bonding polyester to poly(vinyl alcohol) or copolymers thereof, especialy with respect to multilayer, biaxially-oriented beverage bottles.
Other patents which disclose copolyesterethers prepared from 1,4-cyclohexanedicarboxylic acid, 1,4-cyclohexanedimethanol and polytetramethyleneether include U.S. Pat. Nos. 4,003,882; 3,261,812; 3,023,192; 4,221,703; and 3,651,014. U.S. Pat. No. 3,733,909 discloses a PET/copolyesterether copolymer and U.S. Pat. No. 3,907,926 discloses blends of polybutylene terephthalate and copolyesterethers.